The building blocks of population science.
Core concepts — fertility, mortality, migration, and age structure — that underpin everything else on this site.
Terms in this category
The average number of children a woman would have over her lifetime if current age-specific birth rates stayed the same.
TFR is the single most-used indicator of a country's fertility level. A TFR above roughly 2.1 means the population replaces itself naturally; below that, deaths eventually exceed births without immigration.
The number of children (under 15) and older people (65+) compared with the working-age population (15–64).
A high dependency ratio puts fiscal pressure on a society — fewer workers must support more dependants. It is used to assess pension systems, healthcare loads, and long-run growth potential.
How many years it takes a population to double at its current growth rate.
Calculated as 70 ÷ annual growth rate (%). At 1 % growth, doubling takes 70 years. At 3 %, just 23 years. Doubling time illustrates the compounding nature of population growth.
The average number of years a newborn would live if current death rates remained constant.
Life expectancy is a snapshot of current mortality conditions, not a prediction for any individual. It is sensitive to infant and child mortality; improvements there raise the figure sharply.
The age that divides a population exactly in half — half the people are younger, half are older.
Median age rises as fertility falls and life expectancy improves. Countries with a median age above 40 (e.g. Japan, Germany) face workforce shrinkage; those below 20 (e.g. Niger) have high youth shares.
Births minus deaths in a population, excluding migration.
Natural increase can be positive (births exceed deaths) or negative (deaths exceed births, called natural decrease). It is the primary driver of population change for most countries historically.
People arriving in a country minus people leaving, over a given period.
Net migration can offset or amplify natural increase. High-income countries with below-replacement fertility rely on positive net migration to maintain or grow their population.
A chart of how many people are in each age group, split by sex, stacked from youngest at the bottom to oldest at the top.
A classic pyramid shape (wide base, narrow top) indicates high fertility and high mortality — typical of fast-growing populations. A rectangular or inverted shape indicates low fertility and ageing.
Common questions
What is replacement-level fertility?
Replacement-level fertility is the TFR at which a population exactly replaces itself from one generation to the next — roughly 2.1 children per woman in most countries. The "2.1" (rather than exactly 2.0) accounts for the fact that slightly more boys are born than girls, and some children do not survive to reproductive age. Our World in Data explains replacement-level fertility and why the threshold varies between countries.
Why does population keep growing even when TFR falls below 2.1?
This is called population momentum. When a country has a large share of young people, births continue to outnumber deaths even after fertility drops below replacement, because so many women are entering their reproductive years. The effect can last several decades. The Population Reference Bureau's primer on population momentum walks through how and why the lag works.
What is the difference between crude birth rate and TFR?
The crude birth rate counts births per 1,000 people in the total population, so it is influenced by the age structure. TFR standardises for age structure by summing age-specific birth rates, making it comparable across countries and over time.
Why do some countries show negative population growth?
When deaths exceed births (below-replacement TFR and an ageing population) and net migration is also negative or small, the total population shrinks. Several Eastern European and East Asian countries are in this situation.
How is life expectancy different from healthy life expectancy?
Life expectancy counts all years lived. Healthy life expectancy (HALE) counts only years lived in good health, discounting years spent with serious disability or illness. The gap between the two reflects the burden of chronic disease in later life. The WHO Global Health Observatory publishes both life expectancy and HALE by country and year.